An alarm bell that can often be found in military camps and police stations. When this sort of bell was first made, the samurai manning it would ring it when they encountered the enemy or other emergencies, and the resounding noise would travel instantly to every corner of the camp like a peal of thunder.
As the samurai improved their methods of guarding and dispatching personnel, these alarm bells would fall out of common use, but would become a sort of symbol instead.
How to get Headquarters Alarm: Sound of Thunder Furnishing?
The blueprint for Headquarters: Sound of Thunder can be obtained from a Precious chest in giant floating platform after defeating a Thunder Manifestation during the quest Seirai Stormchasers: Part IV.